How to identify Black Granite

To correctly identify Black Granite (Granite (mafic variety)), check each of these features in order. Many rocks and minerals look alike, so cross-reference multiple properties before deciding.

Step-by-step identification

1. 1

   Color

   Overall color and any zoning or banding

   Typically dark gray to black. May exhibit a 'salt and pepper' appearance with white plagioclase and black mafic minerals, or be uniformly dark.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Vitreous to sub-vitreous on fresh surfaces, especially for feldspar and pyroxene crystals. Polished surfaces are highly reflective.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Phaneritic (coarse-grained) for gabbro and diorite, with interlocking crystals visible to the naked eye. Diabase often has an ophitic or subophitic texture. Basalt is aphanitic (fine-grained) or porphyritic.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Individual mineral crystals (plagioclase, pyroxene, amphibole, olivine) are typically anhedral to subhedral, interlocking to form a massive rock.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Minerals within the rock (e.g., pyroxene, amphibole, plagioclase) exhibit characteristic cleavage planes (e.g., pyroxene: two cleavages at ~90 degrees; amphibole: two cleavages at ~56 and ~124 degrees; plagioclase: two cleavages at ~90 degrees). The rock itself does not have a single cleavage plane.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Intrusive mafic rocks (gabbro, diorite) form in large plutons, batholiths, or layered intrusions deep within continental or oceanic crust. Diabase forms in dikes and sills. Basalt forms from volcanic eruptions on the Earth's surface or seafloor.

Key Facts

Hardness

6-7 on Mohs scale (due to constituent minerals like feldspar and pyroxene)

Specific Gravity

2.8-3.3 g/cm³ (denser than true granite)

Crystal System

Monoclinic (pyroxene, amphibole), Triclinic (plagioclase)

Color

Dark gray to black

Luster

Vitreous to sub-vitreous

Transparency

Opaque (in hand specimen)

Fracture

Uneven to conchoidal (for individual minerals), generally irregular for the rock mass

Cleavage

Present in constituent minerals (e.g., pyroxene, amphibole, plagioclase), but not a rock cleavage

Composition

Predominantly plagioclase feldspar (labradorite to bytownite) and mafic minerals such as pyroxene (augite), amphibole (hornblende), and sometimes olivine. Quartz is typically absent or present in very minor amounts (<5%).

Physical characteristics

• Crystal Habit: Granular, interlocking anhedral to subhedral crystals (phaneritic).
• Cleavage Type: Good to perfect in constituent minerals (e.g., two directions at ~90° in pyroxene, two directions at ~56°/124° in amphibole, two directions at ~90° in plagioclase).
• Fracture Type: Irregular to sub-conchoidal.
• Tenacity: Brittle.
• Luster Type: Vitreous to sub-vitreous.

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